1. Field of the Invention
The present invention relates generally to methods for reducing the gas permeability of porous materials. More specifically, the invention relates to the reduction or elimination of gas diffusion through these materials by chemically reacting various chemicals within the porous material to form solids which block diffusion through the porous material. 2. Description of Related Art
Plastic materials are widely used due to their light weight, impact resistance and optical clarity. However, plastic materials are, by their nature, permeable to most gases, including water vapor. There are many uses for plastics where it would be desirable to reduce or eliminate the permeability of the plastic to gases. For example, polycarbonate is used as a substrate and cover for dichromated gelatin holograms in pilot's visors. The light weight, impact resistance and optical clarity of the polycarbonate makes it a desirable material for use in such pilot's visors. However, the inherent gas permeability of polycarbonate allows moisture to be transmitted through the polycarbonate within a few days thereby affecting the dichromated gelatin hologram.
Possible pilot's visors which have been suggested to eliminate moisture contamination of the hologram include visors made from glass, glass-plastic laminates and visors using a vapor deposited glass layer on the surface of the plastic. All of these proposed visors have disadvantages. For example, glass alone is too heavy. Laminated structures in which thin layers of glass (approximately 0.003 inch) are laminated between plastic layers is acceptable for flat substrates but is difficult to work with when it must be spherically or toroidally formed. Further, cracking can occur upon minor impact.
The depositing of a layer of glassy or crystal material onto the surface of a plastic visor in order to reduce permeability has shown some promise. These coatings are typically applied by various vapor deposition procedures, including sputtering and gas phase reactions. Since these coatings are all applied to the surface of the plastic, they are all subject to thermal stresses, abrasion and mechanical stresses. Problems have been experienced with these coatings because they tend to fail under long exposure to high humidity and/or high temperatures. Further, it is difficult to vapor deposit glass as a layer on plastic without the formation of small pinholes.
Attempts to reduce the permeability of plastics are set forth in U.S. Pat. Nos. 4,318,970; 4,329,409; and 4,330,604 patents disclose the use of crystalline and glassy deposited films and the use of thick layers of glass sandwiched between plastic. Also, prestressed plastics have been formed in which solid silicon particles are embedded in a plastic matrix and then subsequently oxidized by diffusing in oxygen gas. This results in a material with reduced permeability, however the material is not optically transparent. Also, the solid silicon has to be embedded in the plastic melt first with the subsequent treatment using oxygen gas to form SiO.sub.2 aggregates. These SiO.sub.2 aggregates are nearly the size of the silicon particles and not molecular in size. This results in a standard "filled" plastic which has the slight advantage of being stressed due to the increased size of the filler. However, the silicon particles are deposited randomly and therefore do not provide optimum reduction in permeability.
The above example is just one of many different situations where it would be desirable to have a plastic material which has been treated to reduce its gas permeability while at the same time not adversely affecting the optical and physical characteristics of the plastic. Accordingly, there presently is a need to provide a simple and effective means for blocking the transmissive "channels" in plastic materials and porous materials in general in order to make the materials suitable for uses where gas impermeability is required.